This invention relates to a process for the production of toughened lenses.
Spectacle lenses and other optical lenses are generally prepared from glass, and they are fatally defective that because of brittleness inherent of glass they are very fragile and frail. Accordingly, development of lenses having improved impact strength for preventing injuries by breakage of spectacles during wearing has been demanded in the art and various attempts have been made to develop such lenses. As means for determining the impact strength in spectacle lenses, a so-called FDA test method was proposed by Food and Drug Administration. According to this method, a lens is supported on a circular stand, a steel ball having generally a diameter of 16 mm and a weight of about 16g is let to fall on the center of the lens from a height of 127 cm, and it is examined whether it is broken or not.
In general, the tensile strength of the surface portion of a lens is drastically reduced by the presence of minute scratches and hair cracks formed on the surface portion during the processing of glass. Accordingly, it is considered that breakage occurs on the surface portion under a tensile stress and the breakage is then developed therefrom. Formation of a toughening layer having an internal compression stress on the surface portion of a lens by subjecting the lens surface to the chemical tempering process is effective for increasing the impact strength as determined by the above-mentioned FDA test method or the like. In lenses which have been subjected to the surface tempering treatment, the effect of preventing breakages owing to defects on the surface portion is satisfactory to some extent, but it has been found that no satisfactory effect can be obtained for preventing breakages developed from the peripheral portion. More specifically, when a lens is subjected to the surface tempering treatment, a tensile stress is generated at the central portion in the direction of the lens thickness in correspondence to the internal compression stress in the toughened layer of the surface portion, but when edge grinding is conducted on the edge portion after the chemical tempering treatment, the toughened layer is removed from the edge portion and a tensile stress layer is exposed. Since the tensile stress in this layer is likely to promote defects in the lens, this tensile stress layer has had influences on the impact strength of the edge portion.